The present invention relates to a process for producing a block copolymer resin which is transparent and has excellent mechanical properties, especially impact strength. More specifically, the present invention pertains to a process for producing a block copolymer resin comprising a vinyl aromatic hydrocarbon and a conjugated diene with employment of an organolithium compound as a catalyst in a solvent principally composed of an aliphatic hydrocarbon in which the rate of addition of the vinyl aromatic hydrocarbon monomer is controlled within a specific range at a stage at which the content of the vinyl aromatic hydrocarbon in the polymer formed has reached a specific value, whereby the block copolymer resin can be produced in a stable solution or suspension.
It has been well known in the art to produce block copolymers having various characteristics by copolymerization of a vinyl aromatic hydrocarbon and a conjugated diene by using an organolithium compound as a catalyst. Especially in case of block copolymer resins containing a relatively higher content of a vinyl aromatic hydrocarbon, it is possible depending upon the polymerization conditions to be employed to obtain block copolymers which are transparent and have excellent impact strength. These copolymer resins tend to find increased demands primarily in the field of packagings and vessels for foods. As the methods for production of these block copolymer resins, there have been proposed various methods, as disclosed in Japanese Patent Publication after examination Nos. 19286/1961, 3252/1972, 28915/1972 and 2423/1973, and U.S. Pat. No. 3,639,517. According to these methods, however, there are used as a solvent for polymerization aromatic hydrocarbons or alicyclic hydrocarbons having greater solubilities for block copolymers with relatively higher contents of vinyl aromatic hydrocarbon. These solvents have greater latent heat for vaporization and hence the polymer can be recovered from the block copolymer resin solution after polymerization only with a large energy consumption. Moreover, such a solvent cannot completely be removed, but remains in a considerable amount in the block copolymer resin finally obtained, to great disadvantage.
For overcoming such a drawback, one can consider to employ an aliphatic hydrocarbon having a relatively smaller vaporization latent heat as a solvent. But a block copolymer resin with a relatively higher vinyl aromatic hydrocarbon content is generally hardly soluble in an aliphatic hydrocarbon and there may be caused various troubles according to conventional methods such as precipitation of masses of block copolymers or deposition of such masses on stirring blades or vessel walls in a polymerization reactor, thereby making polymerization operations extremely difficult. For this reason, an aliphatic hydrocarbon has been unsuccessfully used as a solvent.
Quite recently, as a method for overcoming the problem as described above, there has been proposed a method in which a block copolymer resin is formed as a suspension in a solvent, as disclosed in U.K. Pat. No. 1,457,023. According to this method, in production of a block copolymer resin with use of an aliphatic hydrocarbon as a solvent, a mixture of a vinyl aromatic hydrocarbon and a conjugated diene is polymerized in the first step of polymerization, followed by the second step of polymerization, in which the vinyl aromatic hydrocarbon is subsequently added at a rate below the substantial average polymerization rate at the polymerization temperature. As a result of such a two-step polymerization, there can be formed a block copolymer resin which is suspended in a solvent.
According to this method, however, the rate of the vinyl aromatic hydrocarbon added must necessarily be slow enough, since it is required to add the vinyl aromatic hydrocarbon at the rate below the substantial average polymerization rate, i.e. at the rate permitting no residual monomer to be present in the polymerization system. As a consequence, this method involves the drawback that no improvement of productivity can be expected. As another critical disadvantage, block copolymer resins will readily be separated from the solvent in this method and may be deposited at the bottom of the polymerization reactor under the conditions of insufficient stirring or may become clogged during transfer through the pipes to make transfer of polymers difficult. Thus, this method is impractical in commercial application.
The present inventors have made extensive studies to overcome the problems of prior art in production of block copolymer resins and have developed a process capable of producing a block copolymer resin having good transparency and excellent impact strength in a stable solution of suspension, and thus have accomplished the present invention.